A bank has observed an arrival rate of $\backslash$lambda $=30$ customers per hour. There is only one bank teller on duty with a service rate of $\backslash$mu $=15$ customers per hour. What is the average number of customers in the bank?

a$.$

$3$

b$.$

$3$

c$.$

$0\mathrm{.}667$

d$.$

$150\%$

e$.$

Growing without limit$,$ since $\backslash$lambda is larger than $\backslash$mu so service rate cannot catch up with arriving customers